Articles resilient to projectiles, commonly referred to as armour, are constructed from specialised materials using various methods to impede perforation by projectiles such as bullets and fragments.
Articles of ballistic personal protective equipment (PPE), commonly referred to as body armour, include helmets and vests that contain ‘soft’ flexible armour inserts and ‘hard’ rigid armour inserts: Armour is also used to protect occupants and equipment in land, sea and air vehicles.
Rigid armour inserts, known as Small Arms Protective Inserts (SAPIs), are engineered to protect against high-velocity rifle projectiles and share similarities in materials used and construction techniques with helmets and vehicle armour. Whilst SAPIs are available in many levels of protection, two basic configurations exist.
The first is a 100% fibre based composite SAPI designed to protect against ‘soft’ lead filled Full Metal Jacket (FMJ) projectiles (an example of this is USA NIJ 0101.04 level III—6 strikes of 7.62×51 mm NATO FMJ). The second is a layered SAPI containing a ceramic strike-face to protect against Armour Piercing (AP) projectiles that contain ‘hard’ penetrators (an example of this is USA NIJ 0101.04 level IV—1 strike of 30.06 M2 AP).
100% fibre based composite SAPIs can be manufactured from material such as para-aramid (for example Kevlar® or Twaron®) or ultra high molecular weight polyethylene (UHMWPE) when combined with a suitable resin matrix system. Materials are commercially available pre-impregnated (often referred to as ‘pre-preg’) with resin matrix (such as Dyneema® HB products and Spectra Shield®) to simplify this stage.
A traditional 100% fibre based composite SAPI manufacturing technique is commonly referred to as ‘high-pressure axial pressing’ requiring ‘pre-preg’ material, such as Dyneema® HB product, to be cut into the desired shape and stacked. The stack is then positioned between a matching pair of metallic dies attached within the axial press and compressed concurrently with the application of heat (from electric heating elements or the circulation of hot oil through the metallic dies) to comply with predetermined pressure and thermal cycles. Compression is halted only once the material has cooled sufficiently. If desired, the SAPI is covered using a fabric, adhered with contact adhesive.
It is known that for this type of armour, consolidation under higher pressure equates to higher ballistic performance. DSM, manufacturers of Dyneema® HB products, specify that their material must be consolidated with an axial pressure of at least 180 bar, but preferably 350 bar for a performance increase of approximately 10%. This means that to make a single 10″×12″ SAPI, at least a 140 ton press, but preferably a 270 ton press is required. Thus, if a square meter of armour is required, a 3,570 ton press is needed to consolidate the material at 350 bar. Such presses are expensive and uncommon. Since presses exert pressure in an axial manner, consolidating articles shaped other than flat generate pressure gradients and inconsistent levels of consolidation. Flatter shapes minimise this effect, however items like helmets suffer greatly from uneven consolidation.
Air entrapment within axially pressed SAPIs is an issue. It is common to require between 50-200 ‘pre-preg’ plies to manufacture a SAPI that meets USA NIJ 0101.04 level III standard. When the ‘pre-preg’ is stacked and pressed, air is trapped between the layers and compressed, only to appear as bubbles that indicate areas of delamination when removed from the press. To counter this, pressure must be ramped up incrementally, however this is not ideal and extends cycle time.
A major cost associated with conventionally pressing SAPIs is tooling. Rigid metallic matched die sets are required for all geometries to be manufactured. If a press is large enough to process multiple SAPIs, then multiple tooling sets are required. In addition to the die sets, cooling and heating platens are also required. The production speed for this type of SAPI is slow, since the material has to reach a consistent temperature throughout but not have localised ‘hot spots’ that would permanently damage the material. A typical cycle time is approximately 45 minutes per USA NIJ 0101.04 level III standard SAPI.
A traditional method of manufacturing a layered SAPI containing a ceramic strike-face is to bond a monolithic ceramic tile to the front face of an axially pressed 100% fibre based composite. ‘backing’. A resin or elastomeric compound is usually used as the adhesive material and the assembly is clamped whilst curing in an evacuated bag. The evacuated bag provides an even clamping force of near atmospheric pressure. If desired, the layered SAPI is covered using a fabric, adhered with contact adhesive.
This type of layered SAPI is time consuming to manufacture and can suffer significant through-thickness inconsistency. Typically all kiln fired ceramics exhibit warping, in some circumstances by more than 3 mm. Since the 100% fibre based composite ‘backing’ is shaped by precision machined dies and flexes minimally, the discrepancy in geometry between the ceramic tile and the ‘backing’ is filled by excess bonding adhesive. This adds weight and reduces ballistic performance.
Another traditional method of manufacturing a layered SAPI is a batch-type process that employs an autoclave. Industrial autoclaves are pressure vessels used to process parts and materials which require exposure to elevated pressure and temperature. Typically a stack of ‘pre-preg’ material is placed behind a ceramic tile with an intervening layer of adhesive film or composite. The assembly is then wrapped with a plastic release film and placed in a heat sealable vacuum bag. The bag is then evacuated and sealed. This process overcomes the air entrapment issues encountered during axial pressing and clamps the assembly together to allow handling without any alignment shift. The filled vacuum bags are loaded within an autoclave, which is pressurised, typically 6 to 20 bars, using air or an inert gas and heated to comply with predetermined pressure and thermal cycles. Following sufficient time at elevated temperature, the autoclave is cooled whilst maintaining pressure. Pressure is released when the temperature has reduced adequately. If desired, the layered SAPI is covered using a fabric, adhered with contact adhesive.
The major deficiency of this procedure is low composite consolidation pressure. Autoclaves are pneumatic (gas filled) rather than hydraulic (liquid filled) devices, and are subjected to stringent safety regulations. The typical working pressures that autoclaves use generally range from 6-20 bar, which is well below the minimal (180 bar) and desired (350 bar) consolidation pressures suggested by DSM, manufacturers of Dyneema® HB products. To compensate for this low-pressure consolidation, extra composite is used to achieve ballistic performance; however this increases cost, weight and thickness.